A construction site dating back almost 2,000 years to the supposed disappearance of Pompeii in AD 79. has revealed new evidence of the secret behind ancient Rome’s ultra-durable concrete.
Last year, beneath the volcanic ash that buried Pompeii, archaeologists discovered a completely intact construction site – a rare snapshot of Roman construction work frozen in time.
That site includes piles of neatly organized materials, including the ingredients used to mix the famous durable concrete behind monuments like the Pantheon, whose vast unreinforced dome has stood for millennia.
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New analysis reveals that the secret is a technique that materials researcher Admir Masic of the Massachusetts Institute of Technology (MIT) calls “hot mixing.”
It involves mixing the concrete ingredients directly: a mixture of volcanic ash called pozzolana, along with quicklime, which reacts with water to generate intense heat within the mixture.
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“The benefits of hot mixing are twofold,” Masic said in 2023, when he first discovered the technique through experimentation.
“First, when ready-mixed concrete is heated to high temperatures, it allows chemistry that isn’t possible if you just used slaked lime, producing associated compounds at high temperatures that wouldn’t otherwise form. Second, this elevated temperature significantly reduces setting and setting times because all reactions are accelerated, allowing for much faster construction.”
A third, and crucial, benefit is that the surviving bits, or clasts, of lime give the concrete a remarkable self-healing ability. This could be a major reason why ancient Roman monuments still stand while other civilizations have collapsed.
When cracks form in concrete, they preferentially propagate to lime clasts, which have a larger surface area than other matrix particles. When water enters the crack, it reacts with the lime to form a calcium-rich solution that dries and hardens as calcium carbonate, gluing the crack back together and preventing it from spreading further.
Some of the well organized building materials found on the site. (Pompeii Archaeological Park)
“There’s the historical importance of this material, and then there’s the scientific and technological importance of understanding it,” says Masic. “This material can heal itself over thousands of years, it’s reactive, and it’s extremely dynamic. It’s survived earthquakes and volcanoes. It’s endured under the sea and it’s survived degradation from the elements.”
Although the hot-mix technique provided solutions to the puzzles presented by Roman concrete, it raised a new one: the recipe did not match the description of how the building material was made in the 1 BC treaty. About architecture by the architect Vitruvius.
The Vitruvian method involved first mixing lime with water in a process known as slaking, before mixing the slaked lime with pozzolana. However, this process does not produce the lime clasts observed in real Roman concrete samples.
This mismatch has long puzzled scientists. The writings of Vitruvius represent the most complete surviving documents on Roman architecture and construction. He describes a technique called cement work for building walls, but physical samples from ancient buildings contradicted his instructions.
Pompeii materials put the mystery to bed. Masic and his team used isotope analysis on five of the dry piles of material, identifying pozzolana in pumice and lithic ash, unstrained lime and even lime clasts.
Most tellingly, these dry ingredients have been pre-mixed – an archaeological smoking gun.
Under the microscope, mortar samples from the walls revealed unmistakable signatures of hot mixing: fractured lime clasts, calcium-rich reaction rims that grew into volcanic ash particles, and tiny crystals of calcite and aragonite forming in pumice vesicles.
Raman spectroscopy confirmed the mineral transformations, while isotope analysis showed the chemical pathways of carbonation over time.
“Through these stable isotope studies, we could follow these critical carbonation reactions over time, allowing us to distinguish the hot-mixed lime from the slaked lime originally described by Vitruvius,” says Masic.
“These results revealed that the Romans prepared their binding material by taking calcined limestone (smooth lime), ground [it] to a certain size by dry mixing it with volcanic ash and then adding water afterwards to create a cementing matrix.”
This does not necessarily mean that Vitruvius was wrong—perhaps he described an alternative method of making concrete, or that his work was misinterpreted—but it does indicate that the most durable form of the material had to arise from the hot-mix technique.
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This, the researchers believe, is information that can be incorporated into the way we make concrete, many centuries after the fall of the Roman Empire, leaving its monuments as a reminder not only of its greatness, but also of the ingenuity of its people.
Modern concrete is one of the most widely used construction materials in the world. It also remarkably lacks durability, often crumbling within decades under environmental stress. Its production is also terrible for the environment, requiring a huge resource cost and contributing to greenhouse emissions.
Simply improving the durability of concrete has the potential to make it significantly more durable.
“We don’t want to completely copy Roman concrete today. We just want to translate a few sentences from this book of knowledge into our modern construction practices,” says Masic, who founded a company called DMAT to do just that.
“The way these pores in volcanic ingredients can be filled by recrystallization is a dream process that we want to translate into our modern materials. We want materials that regenerate themselves.”
The research was published in Communication of nature.